The present invention relates generally to an apparatus and method for electropolishing surfaces of metallic objects.
Electropolishing is a method used to obtain a clean and polished surface of a metallic object, and is described for example in McGraw-Hill Encyclopedia of Science and Technology, pp. 810-811, 1982, which is incorporated herein by reference. Typically, electropolishing is achieved by placing the object to be polished (the xe2x80x9cworkpiecexe2x80x9d) in a conductive vessel containing electrolytic solution. A voltage difference is then applied across the workpiece and the vessel, acting as anode and cathode respectively. The resulting current flow within the electrolyte between anode and cathode causes dissolution of the anodic surface and a corresponding deposit on the cathodic surface. Under certain parameters, which may include voltage, temperature, current density, and the composition and viscosity of electrolytic solution, the dissolution of the anode may produce a surface finish on the workpiece which is smooth and polished. Below a certain voltage level, etching may occur. Above the etching voltage level, a constant current region is reached where polishing may occur. At even higher voltage, oxygen evolution may interfere with polishing.
Once the correct voltage is established, various problems may still be encountered which tend to detract from the polish quality of the workpiece surface. One problem is that the current density may be unevenly distributed over the workpiece surface, resulting in an uneven surface finish. It is found that corners or edges with a small radius of curvature tend to attract and concentrate the flow of current in comparison with flat surfaces with a large radius of curvature. Thus, corners or edges of the workpiece may tend to become worn away, while flat surfaces may tend not to achieve the required degree of polish. Further, if the workpiece has a complex shape, current xe2x80x9cshadowsxe2x80x9d may be cast by one element of the workpiece onto another, thus causing uneven polishing of a surface lying in such a shadow. Another problem is that heat is generated during the electropolishing process, and the temperature of the solution may rise during the process if a means for removing such heat is not provided. Generally, the rate and operating voltage of electropolishing are changed by the solution temperature, thereby reducing control over the process. A further problem is that as the process progresses, the opacity of the electrolytic solution may increase due to oxide flakes becoming suspended in the solution, thus impairing visual observation of the workpiece. Various techniques have been developed to reduce the impact of such problems. The anodic workpiece may be continuously rotated in the electrolytic solution, thus providing a more even current distribution across the surface of the workpiece, and reducing current shadows where they might exist. Further, the electrolytic solution may be continuously circulated by draining it from the vessel and pumping it back again, in order to cool it and filter out opaque particles while outside the vessel.
However, despite these techniques for overcoming problems found in the art of electropolishing, these techniques may not be fully effective in overcoming problems of uneven current flow associated with workpieces having an interior surface, such as a tube, because the interior surface may lie within a current shadow no matter how the workpiece is rotated.
Accordingly, there exists a need for an apparatus and method for electropolishing which is capable of overcoming the problem of current shadows which cannot be adequately addressed by rotating the workpiece in the electrolytic solution during the electropolishing process. The present invention addresses these and other needs.
Briefly, and in general terms, the present invention is directed to a new and improved apparatus and method for electropolishing the surface of an object. The apparatus includes a reservoir adapted to direct a steady jet stream of electrolytic solution onto a workpiece through an aperture. A voltage difference is applied across the workpiece and reservoir (as anode and cathode, respectively) while the electrolyte jet stream is directed onto the workpiece, to permit the flow of current through the electrolyte between anode and cathode.
In one embodiment of the invention, the reservoir may be rotated about the stationary workpiece while directing an electrolyte jet stream at the workpiece. In another embodiment, the workpiece may be rotated about an axis and may also be moved linearly on the same axis, while the reservoir remains stationary and directs the electrolyte jet stream at the workpiece. For a workpiece having both an inside and an outside surface, such as a tube, the reservoir may have nozzles or apertures directing electrolyte jet streams positioned both outside the workpiece, so as to direct jet streams at the outside surface, and also inside the workpiece, so as to direct jet streams directly at the interior surface. Desirably, the movement of the workpiece and the reservoir may be arranged to respond to forces controlled by computer or similar automated means, such that rotational and linear movement may be either simultaneous or independent of each other.
After the electrolytic solution has impacted the workpiece the solution may be collected, filtered, cooled if necessary, and then returned to the reservoir for further discharge, thus providing for continuous recycling of the electrolyte.
The apparatus and method of the present invention have the advantage of being able to focus a narrow jet stream of current-bearing electrolyte directly upon an anodic portion of the surface of a complex-shaped workpiece, without interference from current shadows which might be cast by other elements of such a workpiece were the workpiece to be immersed in a vessel of electrolytic solution. Moreover, as the workpiece is not immersed in solution, visibility of the workpiece is not impaired by opaque particles in suspension.
These and other objects and advantages of the invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings of illustrative embodiments.